In the machine processing of various types of tickets, tags, labels, postage imprints and the like, it is generally known to employ detectors which are responsive to colors, and in many cases to the fluorescent emissions of an ink which may be the result of ultraviolet light excitation. Fluorescent inks and dyes and methods of making the same have long been known as disclosed in U.S. Pat. Nos. 2,681,317; 2,763,785; 3,230,221; 3,421,104; 3,452,075; 3,560,238; and 4,015,131. It is known, for example, in the postage meter art to provide a red fluorescent ink for the machine reading of processed mail. Fluorescent colored inks are those in which the ink exhibits a first color, such as blue, black or green, in the visible spectrum and a second color when subjected to ultraviolet light.
Although the inks and dyes used previously for the purposes described above have worked relatively well, certain drawbacks exist. With regard to fluorescent inks, prior fluorescent inks were dispersion inks that experience frequent color separation upon aging. In addition, the prior red fluorescent inks with non-red visual color generally have dull visual colors. It is difficult to obtain a red fluorescent ink with a non-red visual color in solution because of interaction between the dyes that causes the non-red visual color to absorb the fluorescence of the image. This is known as quenching and is occasioned by many factors such as internal conversion, competing mechanisms, absorption of incident light by foreign molecules, solvent interaction, and the like. The quenching effect is obvious with the blue dyes that absorb light between 600-640 nm which is the emission range of the red fluorescent dye.
As generally stated above, machine readable red fluorescent inks have been used in postage meters or franking machines for imprinting postal indicia on a variety of substrates. In the United States, the inks used for printing postal indicia on substrates need to be red fluorescent so that automatic sorting equipment used by the United States Postal Service, which requires inks with red fluorescent characteristics, will perform properly.
The excitation wave length of a typical red fluorescent ink is 254 nm while the emission radiation of the typical ink is typically in the range of 580-640 nm. Postal indicia printed on a typical substrate such as an envelope or label with red fluorescent inks have to have good water fastness, good smear fastness and light fastness. Also, postal indicia printed on various different substrates have to have sufficient contrast properties to enable the indicia to be efficiently detected by the automatic sorting equipment.
Postal indicia or franking machines have been developed that use digital printing technology to permit the use of variable information on the postal indicia and to provide more enhanced printing, resulting in better indicia print quality. Better print quality of the postal indicia enables the postal indicia to be more efficiently read by optical character recognition equipment, bar code readers and other types of machine vision technology typically used in automatic sorting machines and the like. In other words, poor print quality will generally cause Postal Service equipment to erroneously reject postal indicia resulting in increased cost to the Postal Service in the processing of mail.
The use of digital printing technology for printing and franking is restricted, to some extent, by the limited existence of inks that are suitable for franking, and at the same time, are functional with specific ink jet technology. Some magenta inks meet the above criteria. The choice of the magenta dyes is narrow and some of the most brilliant available magenta dyes are fluorescent. One of the few available water soluble magenta dyes is the Acid Red 52. The Acid Red 52 dye has satisfactory solubility in water but a very low water fastness. Thus, a disadvantage of the magenta Acid Red 52 dye is that the ink containing such dye bleeds when exposed to water. Another disadvantage of using current magenta inks is that prints obtained with these inks offset on the back of neighboring envelopes when exposed to water, and the offset prints show an increased fluorescent signal. The dilution of fluorescent imprints on certain envelopes could enhance the fluorescent signal. Therefore, current magenta inks experience back ground offset fluorescence. This causes matter that is not an indicia to appear as an actual indicia printed on an envelope. The above causes Postal Service equipment to erroneously reject postal indicia. Thus, the above effect can cause increased cost to the Postal Service.
Another disadvantage of prior art piezoelectric ink jet inks, which are water-fast, is that the inks use different organic solvents instead of water. Various organic solvents, such as tripropylene glycol methyl ether or other glycol ethers, are used to improve the water fastness. The reason for the above is that the solvents dissolve or disperse colorants that are insoluble in water. Due to safety and compatibility requirements, with various plastic materials used in ink jet print heads, water is nevertheless desirable as the main solvent. The reason why water is desirable as a solvent when plastic materials are used is that water is much less aggressive than the organic solvents. The widely used plastic materials for ink jet printer parts are inker foam, cartridge material, glue, printer base, etc. The above parts may be made for example from acrylonitrile styrene (AS), polymethyl methacrylate (PMMA), and acrylonitrile butadiene styrene (ABS). Therefore, the disadvantage of using organic solvents is that they act as a solvent towards the plastic materials, thereby compromising their strength and causing a potential printer failure. Water is desirable as the main solvent in the ink composition because the plastic ink jet printer components are not soluble therein.
If a fluorescent ink is to be used in an ink jet printer, the fluorescent ink must have certain physical properties, such as a certain viscosity and a specified surface tension. The viscosity of the liquid inks used in current piezoelectric ink jet printers is 1.5-20 centipose (cps) and in the thermal ink jet printer is lower (1-5 cps). The desirable surface tension of liquid ink jet printer inks should be between 30-45 dynes/cm. A disadvantage of current water based ink jet inks is that the ink has a tendency to dry in the nozzles of the printer during operation of the printer and between operations of the printer. One of the properties of an ink that is used in an ink jet printer is the decap time, which is the length of time over which an ink remains fluid in a nozzle opening when exposed to air and capable of firing a drop. Precipitation of the solid in the ink can cause failure or coagulation in the ink and is often due to evaporation of the solvent causing precipitation or crystallization of a solid in the ink at the air/liquid surface. Another disadvantage of the water based inks is that they have to use "naked" (pure) dyes without resins in order to achieve good solubility. The dyes should be preferably direct, acid, basic or reactive. If the solubility of the colorant material in the solvent is not good enough, the drops stability, and the print quality tend to be poor. The long term solubility which affects the shelf life depends on the colorant's solubility in the solvent in various environmental conditions of temperature and humidity. The dyes that exhibit good water solubility suffer from deficiency in water fastness and smear fastness on the generated prints. The foregoing occurs because of the print's solubility in water.
Another disadvantage of dye based inks is that they produce prints with high edge roughness and poorly defined characters which is also called feathering. The foregoing disadvantage is more apparent at lower resolutions such as 240 dpi, where there is less overlap between the dots. Therefore, pigments which are insoluble in water are a preferred alternative to dyes provided that pigments dispersions can be made stable to sedimentation and gelling. The problem of using pigments is that they exhibit fragile equilibrium in dispersion. The dispersions are easily destabilized by changes in temperature, humidity and impurities. Other typical liquid ink properties are the inks ability to penetrate paper and to dry fast by absorption.
Another problem with fast penetrating inks is that the optical density decreases with the ink penetration. The above effect has to be compensated for in order to achieve good print quality. Good print quality may be achieved by using dyes with high absorbency values. This can have an adverse effect on the fluorescent signal (lowering the signal) of the prints due to quenching.
Another problem of prior art inks is that they do not give consistent print quality on a large variety of substrates. Postage meter inks have to be used on a large variety of paper envelopes. The components of the paper can be inhibiting for the fluorescence or can have adverse effects on the optical density of the prints. The wax based inks, such as hot melt ink, do not interact with the paper in the same manner as the liquid inks.
Therefore, there is a need for an ink composition that will have consistent fluorescence and provide an adequate print contrast signal on a broad range of substrates in the sensitivity wavelength range typical of common monochrome scanners.